This paper presents an experimental study and a set of CFD simulations applied to a horizontal tank storing hot water. Hence, the main purpose of this work is to suggest a new and optimal design of the studied horizontal tank which is considered as the main device in individual solar water heaters integrating Evacuated Tube Collectors (ETC) with heat pipes. An experimental study was carried out to measure the maximum temperature of the heat pipes using a thermal camera. Subsequently, CFD simulations were performed using the previously recorded experimental results. The numerical studies aimed to assess the effect of the heat pipe's number, integrated on the lateral internal shell of the tank, on its energetic performances such as the discharging efficiency. In fact, four cases were studied according to the number of the used heat pipes (n). Several performance indicators were defined to identify the optimal configuration, such as the heat transfer coefficient assessed at the heat pipe, temperature contours and streamlines, temperature evolution according to specific paths defined inside the storage tank and the discharging efficiency\ldotsIt was found that the average temperature of the horizontal tank is increased by the rise in the heat pipe's number. On the other hand, the laminar structure of the flow field is disturbed and the outlet temperature of the hot loaded water is also affected. Otherwise, the HTC calculated for the heat pipe close to the outlet of the storage tank for n = 6 achieved 212.5W/K m2, while it is 300 W/K m2 and 230 W/K m2 for n = 8 and n = 10, respectively. Moreover, the discharging efficiency of the horizontal storage tank depends mainly on the instantaneous variation of the water's temperature.